GLTF Export — get scene camera
glTF and three.js do not have Blender's concept of a Scene Camera, so you'll need to use another way to identify which camera you want. Object names are exported with cameras, and Custom Properties are exported as well (if you enable this in the export options). Those would both be available in three.js as camera.name and camera.userData.
Note that the camera may be parented to an empty for export, so you may need to check both camera.name and camera.parent.name for the original name.